For capturing images, an imaging device that converts light into an electric signal has been often used. In general, an imaging device includes multiple pixels and analog-to-digital (A/D) converters. Each pixel includes a photoelectric converter such as a photodiode, a floating diffusion layer, and an amplifying transistor. In the pixel, the photoelectric converter converts light into an electric charge. The floating diffusion layer stores the charge to generate a signal voltage on the basis of a relationship represented by the expression Q=CV. The signal voltage is amplified by the amplifying transistor. The A/D converter converts an analog signal representing the amplified signal voltage into a digital signal. In the expression, Q represents the amount of charge produced by conversion in the photoelectric converter, C represents the capacitance of the floating diffusion layer, and V represents a signal voltage.
In the imaging device having the above-described configuration, a sufficient decrease in capacitance C of the floating diffusion layer sufficiently increases a signal voltage per photon than noise. This makes it possible to determine whether or not one photon is incident. A photon counting imaging device that counts the number of photons and uses the counted number as an image signal has been proposed (e.g., see PTL 1). The photon counting imaging device realizes an extremely high signal-to-noise (S/N) ratio because it is able to completely eliminate random noise and fixed pattern noise caused by analog signal processing.